The slope of the coexistence line between the solid and liquid phases shown in Figure 6.3 is positive. Most pure species behave this way. However, species that expand on freezing (like liquid water to ice) exhibit a negative slope. Using the thermodynamic principles presented in this chapter, show that the slope must be negative.

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P Solid Liquid Critical point Coexistence lines Vapor T Figure 6.3 Identification of the coexistence lines on the PT projection of the PoT surface. Along these lines, two phases can coexist in phase equilibrium.